This invention relates to multiple access communication systems. More particularly, it relates to multiple access wireless local area network (LAN) communication systems.
The need for wireless interconnection of computers operating over LANs has become increasingly apparent as the popularity of LAN usage in office environments has grown. Traditionally, computers and computer related devices operating as part of a LAN have been interconnected using such conventional technologies as twisted pair, coaxial cable and optical fiber cable. A primary goal of conventional systems has been to utilize these technologies to realize fast, reliable, multiple access communications over the LAN. Historically, several communication protocols and topological architectures have been combined with traditional interconnection technologies to realize this goal.
In attempting to achieve the goal of fast communication, the issue of how to make efficient usage of the chosen transmission medium was repetitively addressed by prior art systems. A common trade off encountered with respect to bus allocation is how to allocate bus usage between high duty cycle subscribers and low duty cycle subscribers whose transmissions are characterized by a high peak-to-average transmission rate. Such low duty cycle subscribers are commonly referred to as "bursty" subscribers. Three general approaches were developed; time division multiplexing, frequency division multiplexing and space division multiplexing.
Time Division Multiplexing (TDM) attempts to take advantage of transmission mediums which can support a faster communication data rates then are required by any one user on the network. Essentially, multiple digital communication signals are interleaved in time over a single transmission path. Many different types of TDM based systems have become well known in the art. Examples include polling, priority request, communication path contention, and cyclic time division systems.
In a polling system, a central controller polls remote stations, offering each an opportunity to utilize the communication path. In a priority request system, remote stations request usage of the communication path and a central controller awards control of the path according to priority levels assigned to each station. Contention systems generally involve remote stations transmitting at random times and then retransmitting in the event of a detected collision. In a cyclic time division implementation, devices on the network are only permitted to transmit during a preassigned periodically occurring time slot. Some cyclic time division systems have fixed slot lengths and a fixed period while others allow for these parameters to be dynamically updated. Still others control the length of the time slots by passing a digital code, called a token, between devices operating on the network. Only when a device possesses the token, may it transmit.
Frequency Division Multiplexing (FDM), attempts to exploit transmission mediums which can support a wider bandwidth than is necessary to accommodate the highest frequency baseband device coupled to the network. In an FDM system, each transmission signal is modulated onto a different carrier signal operating about a particular center frequency. Each unique carrier signal is referred to as a channel. One example of frequency division multiplexing is frequency shift keying (FSK). FSK systems may be designed to operate in either a full or half duplex environment. A full duplex implementation assigns separate channels for transmission and reception, thereby enabling transmission and reception to occur simultaneously. Alternatively, half duplex systems require a single channel to be time multiplexed for both transmission and reception. FSK systems generally represent a logical one by a frequency shift in a particular direction relative to the center frequency. Similarly, a logical zero may be represented by an equivalent frequency shift in the opposite direction.
Space Division Multiplexing (SDM) simply adds more signal paths which are spacially isolated from each other, thereby increasing the number of signals that can be simultaneously transmitted. As implemented in a local area network, this involves providing a separated interconnection between any two devices wishing to communicate.
Another common issue raised in attempting to efficiently utilize a transmission medium is whether to implement a baseband or a broadband system. In the context of LANs, broadband refers to any system which transmits the digital information as an analog signal. One example is the FSK approach discussed above. Another example is phase shift keying (PSK). As the name suggests, PSK entails representing a logical one by a signal having a particular frequency and a particular phase while representing a logical zero by a signal having identical frequency but being phase shifted by a predefined amount.
The transmission path multiplexing approaches discussed above have traditionally been applied to various permutations of four topological architectures. These topologies are commonly referred to as; bus networks, loop networks, ring networks and star networks.
A bus network is one of the simplest configurations. In this type of a system the network is typically comprised of a passive transmission bus. The bus may consist of a single cable or many branches. Devices operating on such a network, commonly referred to as subscriber devices, interface directly to the bus and every device has access to transmissions from every other device. Since the bus is passive, it is the responsibility of the devices operating over the network to manage bus access. Traditionally, both frequency and time division multiplexing have been employed on such systems.
A loop network typically consists of an inbound unidirectional signal path, an outbound unidirectional signal path and a unidirectional path coupler for transferring signals from the inbound path to the outbound path at the system head-end. A loop network may also be a passive system. However, in some prior art implementations, a digital bus repeater (DBR) is employed as a path coupler. A DBR provides certain minimal formating functions and regenerates signals tending to deteriorate over long signal paths. Devices operating on a loop network typically interface to the network via an appropriate bus interface unit (BIU). As in the case of bus networks, both frequency and time division multiplexing have been utilized for efficient transmission path allocation.
A ring architecture, in its most rudimentary form, is a unidirectional closed loop signal path. Typically, devices may interface to the ring either directly or through an appropriate BIU. In some prior art implementations DBRs are inserted into the signal path periodically around the ring, and each subscriber device interfaces to the ring via a DBR. In other prior art implementation, two concentric unidirectional rings are employed. In an analogous fashion to the loop network, one ring services incoming communications signals while the other services the outgoing. Since the ring is a predominantly passive network, the devices operating over the network are responsible for efficient network allocation.
A star network typically involves having each subscriber device connected, via a dedicated communications path, to a central communication controller. Traditionally, if one device on the network wishes to send a message to another device on the network, it transmits the message to the central controller and the central controller, in turn, redirects the transmission to the appropriate destination device. In other prior art implementations of star networks the central controller acts as a switching unit. As such, instead of relaying communications, upon request from a transmitting device, it Performs switching operations to physically connect a transmitting device to a specified receiving device. As can be seen, the central controller relieves the subscriber devices of responsibility for efficient network allocation.
Many common prior art implementations have been developed which employ contention type time multiplexing protocols in an attempt to realize the most advantageous usage of the above discussed topologies. One early implementation, capable of utilizing either a bus, loop or ring topology can be characterized as a free-for-all system. According to this early implementation, subscriber devices transmitted at random and then waited a period of time for an acknowledgment from the destination device. If no acknowledgment was received then the sending device retransmitted. A substantial deficiency of this system was the number of communication collisions caused by contending subscriber devices.
Subsequent prior art implementations improved on this approach by adding a carrier sensing feature. Generally referred to as carrier sense multiple access (CSMA) or listen before talk (LBT), this protocol required transmitting devices to determine whether the shared communication path was available prior to beginning a transmission. However, this protocol, nevertheless, suffered inefficiencies because collisions of simultaneously transmitted signals went undetected until the transmitting device failed to receive the expected acknowledgment from the receiving device.
The CSMA approach was later improved upon by the addition of collision detection (CSMA/CD), also known as a listen while talk protocol (LWT). According to a LWT protocol, the transmitting device monitors the bus during transmission so that a collision is detected at the earliest possible time. Considerable time savings occur by early detection if transmission distances are such that there is a long round trip delay between the transmitter and the receiver. Additional time savings occur when lengthy communications are involved and a collision occurs early on in the transmission.
Several non-contention based prior art systems have been developed which relieve subscriber devices operating on the network from having to detect collisions. One such implementation which tends to eliminate communication collisions is a cyclic TDM approach, commonly referred to as a token passing ring. As the name suggests, this network exploits a ring type architecture. In a token passing ring system, the devices on the network continuously pass, between themselves, a digital code, called a token. Only when a device possesses the token, may it transmit data over the ring. In this way network communications are interleaved in time. FDM may also be utilized to modulate the baseband signals being time multiplexed over the communication path.
Two other implementations of cyclic TDM systems are well known in the art. They are fixed slot allocation and dynamic slot allocation. In a fixed slot allocation system, regularly occurring time slots in a repetitive framed sequence are dedicated to specific devices operating on the network, for their transmissions. In dynamically allocated systems, Parameters such as the size of each time slot and the number of time slots allocated to a particular device may be varied. Since a device only transmits during its allocated time slots, communication collisions generally do not occur.
Although the above discussed prior art implementations have succeeded in realizing fast, reliable, secure communications in a multiple access LAN environment, they nevertheless suffer from substantial deficiencies. High on the list of drawbacks to a hardwired system is the considerable cost of installation resulting from having to run cables throughout an office. Another deficiency is the difficulty associated with relocating subscriber devices within an office. A further drawback of hardwired systems is the cost and difficulty associated with moving an entire system from one location to another. Therefore, the most recent activity in the field of LANs has centered around the development of wireless LANs.
There are two primary technologies being developed to provide wireless LAN connections. One is infrared light and the other is radio. Most systems that propose the use of light to transmit data over a local area networks focus on wavelengths in the infrared part of the visible spectrum. This choice is natural because the physical devices for transmitting are plentiful and inexpensive. Additionally, infrared technology is also suited to very high data rates.
However, infrared communication systems are mostly limited to line-of-sight links because light cannot penetrate doors and walls. Systems which use diffused sources and which rely on reflections off walls and ceilings have been built, but none have succeeded in multiple-room environments without the use of extensive repeater networks. Additionally, infrared tranceivers are easily interrupted by people walking around an office.
Radio waves have long been used for voice communication. Hand-held walkie-talkies are essential for plant-maintenance personnel in large office buildings. FAX and telephone communications over cellular radio links are becoming integral to many modern businesses. The use of radio technology for high speed data communication over such links is now emerging.
The primary advantage that radio has over infrared is its ability to penetrate most solid objects found within a building. Consequently, objects such as wall boards, wood doors, modular office walls, and people will not disrupt a radio signal to the same degree as an infrared signal. However, prior art systems have yet to completely overcome several difficulties inherent to designing a radio based LANs for operation in an office environment.
One significant difficulty is effectively dealing with multiple signal paths created by transmission signal reflection. Other issues involve guarding against unauthorized reception of communication signals and interference with those signals.
The FCC has recognized the need to use radio transmissions for commercial in-building communication systems and has allocated three separate bands for low powered systems that do not require user licensing. Specifically, these frequency bands are 902-928 MHz, 2.4-2.5 GHz, and 5.8-5.9 GHz. The 902-928 MHz band is both fairly narrow and crowded with such devices as store security systems and paging systems. However, the other two bands offer sufficient bandwidth to transmit data at LAN megabit data rates and they are virtually free of interference from other devices.
Another frequency band that is being used for wireless networks is 18-19 GHz. This frequency range does offer a great deal of bandwidth, ten 10 MHz channels. However, the use of this band requires an FCC license on a per location basis. This restricts the freedom a user has in installing a wireless LAN as well as the ability to move the system. Connection costs are significantly higher because the cost of radio components increases along with the frequency.
Another drawback to utilizing this band is that it is very close to infrared and as a result, shares many of the propagation characteristics of light. Since signal penetration of solid objects, such as walls is low, the effective range is small, typically a 40 foot radius. This increases the need to provide many repeaters to cover a typical office environment and further increases the overall system cost.
Accordingly, it is an object of the present invention to provide a wireless, radio frequency, multiple access communication system for use within an office environment.
It is a further object to provide a wireless, radio frequency, multiple access communication system which is effectively immune to information transmission problems caused by signal reflections.
It is another object of the invention to provide a wireless radio frequency, multiple access communication system which accommodates both high and low duty cycle subscribers.
An additional object of the invention is to provide a wireless radio frequency, multiple access communication system which implements a carrier sense multiple access with collision detection protocol.
Another object of the invention is to provide a wireless system which avoids any need for FCC licensing, thereby providing the user with additional flexibility with respect to system installations and subsequent system relocation.